Abstract out (forward) jump threader state handling.

Abstract out (forward) jump threader state handling.

The *forward* jump threader has multiple places where it pushes and
pops state, and where it sets context up for the jump threading
simplifier callback.  Not only are the idioms repetitive, but the only
reason for passing const_and_copies, avail_exprs_stack, and the evrp
engine around are so we can set up context.

As part of my jump threading work, I will divorce the evrp engine from
the DOM jump threader, replacing it with a subset of the path solver I
have just contributed.  Since this will entail passing even more
context around, I've abstracted out the state handling so it can be
passed around in one object.  This cleans up the code, and also makes
it trivial to set up context with another engine in the future.

FWIW, I've used these cleanups and the path solver in a POC to improve
DOM's threaded edges by an additional 5%, and the overall threading
opportunities in the compiler by 1%.  This is in addition to the gains
I have documented in the backwards threader rewrite.

There are no functional changes with this patch.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-dom.c (dom_jump_threader_simplifier):
	Put avail_exprs_stack in the class, instead of passing it to
	jump_threader_simplifier.
	(dom_jump_threader_simplifier::simplify): Add state argument.
	(dom_opt_dom_walker): Add state.
	(pass_dominator::execute): Pass state to threader.
	(dom_opt_dom_walker::before_dom_children): Use state.
	* tree-ssa-threadedge.c (jump_threader::jump_threader): Replace
	arguments by state.
	(jump_threader::record_temporary_equivalences_from_phis):
	Register equivalences through the state variable.
	(jump_threader::record_temporary_equivalences_from_stmts_at_dest):
	Record ranges in a statement through the state variable.
	(jump_threader::simplify_control_stmt_condition): Pass state to
	simplify.
	(jump_threader::simplify_control_stmt_condition_1): Same.
	(jump_threader::thread_around_empty_blocks): Remove obsolete
	comment.
	(jump_threader::thread_through_normal_block): Record equivalences
	on edge through the state variable.
	(jump_threader::thread_across_edge): Abstract state pushing.
	(jt_state::jt_state): New.
	(jt_state::push): New.
	(jt_state::pop): New.
	(jt_state::register_equiv): New.
	(jt_state::record_ranges_from_stmt): New.
	(jt_state::register_equivs_on_edge): New.
	(jump_threader_simplifier::jump_threader_simplifier): Move from
	header.
	(jump_threader_simplifier::simplify): Add state argument.
	* tree-ssa-threadedge.h (class jt_state): New.
	(class jump_threader): Add state to constructor.
	(class jump_threader_simplifier): Add state to simplify.  Remove
	avail_exprs_stack from class.
	* tree-vrp.c (vrp_jump_threader_simplifier::simplify): Add state
	argument.
	(vrp_jump_threader::vrp_jump_threader): Add state.
	(vrp_jump_threader::~vrp_jump_threader): Cleanup state.